It is well known that a light-modulating device for controlling electric image signals in relation to a beam intensity has been playing a key role in an image-reproducing apparatus of the scanning type. As such a light-modulating device, for example, there a light modulating discharge tube has been commonly used. Quite frequently a light modulator has been utilized in combination with laser beam. When an ultrasonic modulator is utilized as such a light modulator for a laser beam, it is widely recognized that its modulation characteristic does not demonstrate a good linearity of modulated beam since the output beam intensity is not directly proportional to the level of input signals. Further, it has been pointed out as a fatal drawback that it is is very difficult to obtain a stable modulated beam, since the laser beam source itself is very likely to fluctuate.
These problems with respect to the linearity and the stability of modulated beam have heretofore been dealt with separately. For instance, in order to improve the linearity, a functional transducer has been commonly used wherein the synthesized approximation is carried out by a diode or the like has been commonly used. In this method, however, it has turned out to be difficult to make suitable adjustments and it is extremely susceptible to the change of temperature.
On the other hand, the improvement of the stability has usually been carried out either by stabilizing the laser beam source itself or by feeding two kinds of ultrasonic waves to the ultrasonic modulator; one of such ultransonic waves is to be used for modulating the laser beam while the other is used as a negative feedback signal for securing the stability so as to curb the fluctuation of the laser beam source.
However, these known methods naturally require very complicated apparatus, which are therefore very expensive ones. Particularly in the latter method, not only is it necessary to feed two kinds of ultrasonic waves to the modulator but also to provide separate optical systems for picking up a modulated beam used for reproduction and another beam used for the negative feedback signal. These necessities, needless to say, will make the optical systems and a drive circuit of the light modulator more complex, and, what is more, the problem as to the linearity of a modulated beam still remains unsolved.
Therefore, the prime object of the present invention is to provide an apparatus for improving the stability and simultaneously the linearity of a modulated beam with respect to image singals, which beam is to be used as the exposing beam in the reproduction of an original image.